Diverting a VLAN before routing.

[professorguy]

I have a distribution switch (a 3750 with "no ip routing") which has an incoming dot1q trunk carying vlans 17, 18 & 19. It has a trunk up to the core carying only 18 & 19. It has an access port on vlan 17.

Here's a clip:

interface Gi1/0/1
description Trunk from floor
switchport trunk encapsulation dot1q
switchport trunk allowed vlan 17,18,19
switchport trunk mode
!
interface Gi1/0/2
description Trunk to Core
switchport trunk encapsulation dot1q
switchport trunk allowed vlan 18,19
switchport trunk mode
!
interface Gi1/0/3
description VLAN 17 Server
switchport access vlan 17
!

Will packets tagged as 17 coming over the trunk get repeated (de-tagged) down to the access port? Or will they have to be routed there by the core?

 

[KiscoKid]

They will be de-tagged and passed to the access port. They should not be passed to the upstream core switches if VTP pruning has been enabled thereby preventing the switch from forwarding traffic for VLAN 17 as it's not in use on the remote core switch.

 

[professorguy]

My understanding is that VTP pruning is necessary only if the trunk to the core includes vlan 17. It would be automatically pruned since the core has no vlan17 access.

But I am specifically not including vlan17 on the trunk to the core so perhaps no pruning is necessary? In essence, I am doing a manual prune, which is perfectly OK with me. Our network is relatively small (a few routers, a couple dozen switches, a few hundred clients) and I want manual control over every device, port and route in the place.

 

[KiscoKid]

You are correct. If you have manually removed VLAN 17 from the trunk to the core, then VLAN 17 traffic will not be passed over it.